Neptune: New Discovery from Old Data

by Paul Gilster on June 30, 2011

If you’re trying to figure out how fast a gas giant rotates, you have your work cut out for you. Jupiter seems to present the easiest case because of the famed Red Spot, first observed by the Italian astronomer Giovanni Cassini. But gas giants are thought to have a relatively small solid core, one that is completely obscured by their atmospheres. Rotation involves atmospheric effects as the gases slosh and swirl. No wonder astronomers were glad to find Jupiter’s pulsating radio beams, discovered in the 1950s. Rotation of the planet’s inner core results in a magnetic field that produces these signals, offering our best estimate on the planet’s actual rate of rotation.

We now know that the largest of the planets is also the fastest rotating, completing one rotation every 9.9 hours. But even this turns out to be an average because the gaseous nature of the planet causes it to experience differential rotation. Head for the poles and you find a slightly slower rotation period than you do at the equator. The rotation speed of the Jovian magnetosphere is usually the rate cited, but Jupiter teaches how tricky gas giants can be. The equatorial bulge caused by its swift rotation even makes it tricky to measure the planet’s diameter, which will vary depending on whether you measure it from the equator or the poles.

The Problem with Neptune

University of Arizona planetary scientist Erich Karkoschka had to keep such issues in mind when he went to work on the rotation rate of Neptune, another planet whose surface is shrouded by a thick atmosphere. The scientist went on to analyze publicly available images of Neptune from the Hubble Space Telescope archive, studying 500 of them to record details of the atmosphere and track distinctive features over long periods of time. Two features in Neptune’s atmosphere drew his attention, rotating five times more steadily even than Saturn’s hexagon, an atmospheric feature previously thought to be the most regularly rotating feature of any of the gas giants.

The two Neptunian features are known as the South Polar Feature and the South Polar Wave, and the odds favor their being vortices in the atmosphere similar to Jupiter’s Red Spot. Using the Hubble imagery, Karkoschka was able to track them over the course of twenty years. He arrived at a rate for Neptune’s rotation that he believes to be 1,000 times better than earlier estimates. The regularity was remarkable: Both features appear exactly every 15.9663 hours, with less than a few seconds of variation. The regularity suggests these features have some kind of connection to Neptune’s interior, possibly the result of convection driven by warmer and cooler atmospheric areas, but at this point, the exact process remains to be determined.

Image: In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Credit: Erich Karkoschka).

Karkoschka’s next move was to home in on Voyager imagery from 1989, which offers resolution higher than the Hubble instrument, searching the area around the two identified features:

I discovered six more features that rotate with the same speed, but they were too faint to be visible with the Hubble Space Telescope, and visible to Voyager only for a few months, so we wouldn’t know if the rotational period was accurate to the six digits,” the scientist added. “But they were really connected. So now we have eight features that are locked together on one planet, and that is really exciting.”

We have a great deal to learn about the interiors of gas giants, but this work is a step in the right direction. We have a measure of Neptune’s total mass. What we lack is information on how it is distributed. Karkoschka again:

“If the planet rotates faster than we thought, it means the mass has to be closer to the center than we thought. These results might change the models of the planets’ interior and could have many other implications.”

Saturn’s Rotational Challenges

A bit closer to home than Neptune, Saturn reminds us how quickly adding to our data can overturn established estimates. Voyager 1 and 2 found radio signals as they flew past Saturn that could be clocked at exactly 10.66 hours, seemingly iron-clad evidence of the planet’s rotation period. Then Cassini arrived. And while we were all caught up in the visual splendor of the scenes the spacecraft sent back, its sensors were detecting a change in the period of the radio signal of about one percent that could not have been the result of a change in rotation. Cassini went on to discover that Saturn’s northern and southern hemispheres seemed to be rotating at different speeds, another puzzle to add to the gas giant rotation problem.

We thus learn that the radio signals originating with Saturn’s magnetic field are not quite as reliable as we had thought, lagging behind the planet’s core as the interior rotates and drags the magnetic field with it. The sloshing gases of the outer planets continue to confound us, though the work of Karkoschka and others is helping us to pin down the problem areas. The new work is also a splendid example of how much science can be done with publicly accessible data, items long filed away that are ripe for further analysis and may contain clues to such mysteries.

The paper is Karkoschka et al., “Neptune’s Rotational Period Suggested by the Extraordinary Stability of Two Features,” in press at Icarus (abstract).

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{ 9 comments }

kurt9 June 30, 2011 at 14:39

400m/s? That’s a hell of a wind!

Adam June 30, 2011 at 16:58

Hi Paul
Thanks for commenting on this one, though I do wonder if the rotation isn’t actually slower – some recent work suggests Uranus is quicker and Neptune tardier than their radio emissions imply. Similarly, as you note, Saturn’s under renewed scrutiny too.

Daniel Suggs June 30, 2011 at 17:20

Paul,
“But gas giants are thought to have a relatively small solid core,” I’ve asked others this question before and I’m just wondering if you have any thoughts on it: Could Mercury be the core of a former gas giant that has lost it’s atmosphere to the Sun? Is there anything which could or does prove or disprove this? It seems many extrasolar systems have large gas giants orbiting in close, why couldn’t there have been one here?
Thanks for the good reads. Dan

andy July 1, 2011 at 2:47

Daniel Suggs: The short answer is no. Mercury is too far from the Sun for significant mass loss from a gas or ice giant planet. If Mercury had formed as a gas giant, it would still be a gas giant today.

Even for worlds like CoRoT-7b and Kepler-10b with orbital periods less than a day (compared to Mercury’s 88 days) it is not entirely clear that these planets could be produced by evaporating a gas- or ice-giant planet. See this abstract.

Ronald July 1, 2011 at 6:13

Daniel: the key word here is *relatively*. If I am not mistaken, the core of a gas giant is still some 5 to 10 earth masses, in order to enable the accumulation of the massive gas envelope.
So Mercury is really (too) tiny.
Furthermore, it is the innermost planet. If it had ever been an inwardly migrating gas giant I doubt that there would have been 3 other terrestrial planets in stable orbits.
What might be a real possibility in this context though, are the recently found super-earths in close orbit, which have been suggested by some to be (at least in some cases) the remaining cores of inwardly migrated gas giants. On the other hand, these super-earths may also have formed in situ, not allowing for much gas accumulation and gas giant formation because of their proximity to the star.
It would be very fascinating to be able to distinguish between these two origins, however I would not know whether this is presently possible.
Anyone else?

Paul Gilster July 1, 2011 at 9:17

Daniel Suggs, good question, though I think andy has it right that we’d still see a gas giant in that orbit if Mercury had ever been one. Most of the exoplanetary ‘hot Jupiters’ we observe are in extremely tight orbits, well within the orbital distance of Mercury.

Daniel Suggs July 1, 2011 at 20:26

Thanks to all for the good answers. Dan

Rob Henry July 3, 2011 at 18:12

I have two questions to follow Daniel Suggs’. How small can a rocky planet be and still accumulate a massive atmosphere under the most unusual, yet still plausible circumstances? I know that this depends on the temperature, density and longevity of the gas cloud from which it is condensing, but do not know how to place limits on these parameters.

The other question is what limits can be placed on the magnitude of the T-Tauri stage of our sun? I note that the sun would seem to have nothing to balance its collapse and release of heat against until thermonuclear ignition. I also note that every paper I have read on this stage gives very different estimates but, from memory, none places any upper limits on it.

In short is there still a 0.01% chance that Mercury is the core of a lost giant?

Adam July 5, 2011 at 3:08

Hi Rob
The simple answer is maybe.

The more complicated answer is that the discovery of exoplanets has taught us not to make too many generalizations from our own solar system and our theoretical prejudices. So it doesn’t seem likely that Mercury is a Jovian core, but presently it can’t be ruled out totally.

Recently Sergei Nayakshin and Aaron Boley have independently given good reasons for thinking the terrestrial planets, at least Earth & Venus, might’ve once been Jovian mass proto-planets. A proto-planet is a gravitationally bound mass of H/He and ‘metals’, but it has to lose a lot of entropy before stabilizing as a planet proper. Thus it’s more diffuse and liable to tidal disruption by the central star – which they suggest happened to Earth and Venus, and maybe the smaller terrestrials. Only further work and observational tests can tell us which scenraio is correct.

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